Barney's Blog no 2 - Locating Missing Manhole Covers (Part 2)

Public authorities sometimes seem to ignore common sense on construction projects and not consider costs of moving utilities in their plans, or forget that there are utilities underground.

Sewers and drains are generally the deepest utility, and the cost to move them due to the deep excavations necessary is high. Discovering them halfway through a project when a foundation pile is drilled through a main sewer is a disaster to be avoided. Generally the construction company's contract will define responsibility, but if it does not the subsequent legal costs makes sure that everyone pays.
So it is important to get all the existing site information before design work begins; if not construction costs increase.

If construction projects are or reclaimed land or "brownfield sites", often drain and sewer routes run underneath, site has been filled with all kinds of rubbish, and new connections into old drainage may be difficult.

Searching for a lost manhole on an old site using a metal detector is difficult and time consuming because existing scrap metal parts confuse the search.

The more efficient way is to locate the sewer line using a Vivax sonde or the sonde fitted to the vCam 5 drain camera. Then the route of the sewer/drain can be accurately marked out, by walking that line with the metal detector, the lost cover can be located and location time is reduced, providing that the cover can be detected by the metal locator.

Simple enough; but what if the cover is deeper and outside the range of conventional metal detectors? There are several special magnetometer type locators that only detect ferrous metals, such as the Metrotech FM880B and Schonstedt that have great depth capability and these types work very well for lost covers perhaps down to 2M depth; but in reinforced routes, areas with various ferrous metal scrap these are difficult to use as they also detect the ferrous scrap metal parts.

OK, so how do we solve the problem if the cover is deep buried in a area of made up ground and rubbish?
There is a little known technique using the drain sonde, either in the CCTV system, or one of the Vivax sondes.

Everyone knows that the normal sonde with the 33kHz frequency will not radiate any signal if it is inserted into a cast iron sewer pipe, and the user needs to switch to a low frequency sonde to have any chance of successful location; so in the same way, if the normal sonde passes into a manhole chamber with a iron or steel cover, the signal from the sonde will be lost on the ground above.

So the successful location technique will be to have one operator feeding the flexrod with sonde or camera rod along the drain/sewer and the other locating the sonde on the surface. Good cooperation and a system of hand signals or 2-way radios is needed between locator user and operator feeding-in the sonde. The sonde feeder pushes about 2-3 Metres at a time, and the locator user locates the sonde after each push. When the sonde signal reduces, the sonde is somewhere under the missing cover. By moving the sonde backwards and forwards to get the best cut off of the signal, it is possible to "focus the shadow" that the manhole cover makes.
Generally a cover closer to the sonde than the surface will cast a wider signal shadow, and a sonde closer to the surface will cast a narrower signal shadow, but of course that depends on the size of the cover.

And that reminds me of a story....
Not far from where I live, There is a large secondary school that has about 650 students. The local authority and education authority were trying to reduce costs (as always) and considered that if they levelled the games playing fields they would need less staff to supervise the students. There were 6 football fields on 3 different levels because of the local terrain.
So they went out to tender for a land levelling project, at the same time building up and filling the lower parts with all sorts of rubbish and metal scrap to save on costs of concrete and earth. The higher parts were scraped down to spread over the lower parts, and finally the whole top surface was coated in a fine topsoil and levelled ready for planting grass in springtime. The whole project was supervised by a specialist in landscaping and grass, nobody had told him about drains, and he suddenly realised 2 days before several tons of grass seed arrived, that the school had drains, drains had manhole covers and 650 students need the bathroom.

The manhole covers had been buried and forgotten, but the drains were still there, but nobody knew exactly where they were.
We were called in to locate the manhole covers. Impossible to use a metal detector, as it found every piece of scrap metal.
After considerable discussion with the local authority, afraid to admit that they were wasting public money, we had one manhole dug up so that we could use a sonde on the end of a flexible fibreglass rod to locate the precise sewer route and find the covers using the technique of the sonde signal being shielded by a metal cover. The second manhole was dug up and found 3 M deep. From here they reinserted the flexrod and sonde until all the known covers had been found. At each stage a new manhole had to be reconstructed above the old one, and the ground re-levelled after completion, a week of struggle to find all the covers and considerable unplanned expense, paid for out of public taxes.
This story must have a moral somewhere...